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Old 21st June 2008, 01:47 PM   #1
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Default insulation from heatsink help please

I think I just had one amp fry because the TO220 made contact with the heatsink (its trashed so I'm not sure). I have an identical amp that I used to diagnose the first. When I bolted it all together and tested it, it started to dump current in a big way. I took it apart again but couldn't find a fault. I used all the best parts to rebuild and it seems ok now. I wonder if I had over tightened the fixing screws. I'm scared to death that it might all be down to "feel".

What failsafe precautions can I take to make sure it stays insulated?
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Old 21st June 2008, 03:47 PM   #2
Minion is offline Minion  Canada
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Maybe instead of useing a Fastening bolt use a bar that goes across all the IC/transistors and holds them against the heatsink without touching the Tab of the Transistor...I use this method from time to time and it works very well....


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Old 21st June 2008, 04:32 PM   #3
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Minion has a good idea and have seen this done before with good success.

You will need a fairly heavy aluminum bar to clamp the transistors to the heatsink. The bar cannot bend or deflect when fastened, or the thermal transfer of each transistor to the heat sink will not be equal.

I've had VERY good results using Bergquist Sil-Pads. Excellent thermal transfer and electrical insulating. No goop or mess. You can also trim them with a scissors.

Here's a link to one of the to-220 patterns on Digikey. http://search.digikey.com/scripts/Dk...name=BER107-ND

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Old 21st June 2008, 04:56 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
ensure the burr around the heatsink hole is removed.
Ensure the To220 face is flat.
Use an insulating top hat washer between the bolt and the To220 tab.
Ensure that the heatsink to tab insulation is undamaged and in the correct position while tightening up.
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Old 21st June 2008, 05:35 PM   #5
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"You will need a fairly heavy aluminum bar to clamp the transistors to the heatsink. The bar cannot bend or deflect when fastened, or the thermal transfer of each transistor to the heat sink will not be equal."

U-shaped channels can be used too. I've seen steel used in an amp, should work nice as it is harder than aluminium. This was also an U-channel. There isn't really much to be gained by trying to extract heat through the top of the transistors so it won't matter that it has worse thermal performance.
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Old 21st June 2008, 08:52 PM   #6
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Default Re: insulation from heatsink help please

Quote:
Originally posted by Pbassred
I think I just had one amp fry because the TO220 made contact with the heatsink (its trashed so I'm not sure). I have an identical amp that I used to diagnose the first. When I bolted it all together and tested it, it started to dump current in a big way. I took it apart again but couldn't find a fault. I used all the best parts to rebuild and it seems ok now. I wonder if I had over tightened the fixing screws. I'm scared to death that it might all be down to "feel".

What failsafe precautions can I take to make sure it stays insulated?

1/ Make sure there is absolutely no burr around the holes.
I always countersink my holes to be 100% sure.
2/ Tighten down the bolt holding the transistor until tight but not until it gets close to stripping the threads.
3/ Use flat metal washers to spread the load with a locking washer on top of the flat washer.

4/ Dont use too much thermal paste as it can hydraulic and after a while the pressure goes and can loosen the bolt holding the transistor ! Spread it thinly.



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Old 21st June 2008, 09:57 PM   #7
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One should definetly use some kind of spring washer so that thermal cycling or the paste compressing won't make the transistors come loose.
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Old 22nd June 2008, 08:36 AM   #8
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Cheers,
I'll try being burr free with a spring washer for now, but I'll definatly work on clamps later. When I worked for Tyco our power supplies were designed like that.
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Old 22nd June 2008, 09:20 AM   #9
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Take care with top hat style plastic insulators for the screws, the last lot I got from RS the length of the stem part was longer than the thickness of the transistor, so I had to trim some off else it crushed up and didn't hold the transistor too well.
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Old 22nd June 2008, 12:58 PM   #10
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Does anyone know where to find insulators for TO-264 cases? (like for MJL2119x, 2SC5200, FJL4215 etc.) I looked at digikey and found these for TO-247:
http://search.digikey.com/scripts/Dk...name=BER178-ND

The insulator is the same size as a TO-264 - not optimal as there should be some margin. Is it possible to get mica insulators for these?
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